The present invention relates to a three-stage cross-connect switch system, and more particularly to a method for setting branch routes to the three-stage connect switches constituting this three-stage connect switch system.
Each of a plurality of nodes involved in a communication network is provided with a switch, known as a cross-connect switch, for setting a route to connect any one input line to any one output line. This cross-connect switch is a non-blocking switch which makes it possible to connect any one vacant input line to any one vacant output line without obstructing the connection status of any route already set between any other input line and any other output line, i.e. without requiring a rearrangement to alter any existing route. One example of such non-blocking switch is proposed by C. Clos in his article entitled "A Study of Non-blocking Switching Networks" in The Bell System Technical Journal, March 1953, pp. 406-424. This proposed non-blocking switch is commonly known as the Clos switch, which can meet in a non-blocking state a request for one-to-one connection of one input to one output (usual connection) when the number m of intermediate or middle stage switches satisfies the necessary condition of m.gtoreq.n+g-1 where n is the number of inputs (input lines) to each input stage switch and g is the number of outputs (output lines) from each output stage switch in a three-stage basic configuration.
Four routing strategies for finding one or more intermediate switches and the condition for the number of intermediate stage switches required to maintain a non-blocking state in setting multiconnection routes for such a three-stage Clos switch system are proposed by F. K. Hwang in his article entitled "Three-stage Multiconnection Networks which are Nonblocking in the Wide Sense" in The Bell Technical Journal, December 1979, pp. 2183-2187. Multiconnection is classified by the form of connecting the input and the output into branching connection (connecting one input line to a plurality of output lines) and confluent connection (connecting a plurality of input lines to one output line), and both are functions usually required by a switching network for the connection of the same sound source or for simplified conferencing. A cross-connect switch requires branching connection to set an alternate route between nodes for use when trouble has occurred on the transmission path or on a similar occasion. If it is intended to maintain a non-blocking state by using Hwang's proposal in a cross-connect switch which requires such branching connection, a greater number of intermediate-stage switches than in the three-stage Clos switch system will inevitably be required.